Re: A couple of TSC questions

[Feng Tang]

On Fri, Mar 24, 2023 at 05:47:33PM -0700, Paul E. McKenney wrote:
> On Wed, Mar 22, 2023 at 01:14:48PM +0800, Feng Tang wrote:
> > Hi, Paul
> > 
> > On Tue, Mar 21, 2023 at 04:23:28PM -0700, Paul E. McKenney wrote:
> > > Hello, Feng!
> > > 
> > > I hope that things are going well for you and yours!
> > 
> > Thanks!
> > 
> > > First, given that the kernel can now kick out HPET instea of TSC in
> > > response to clock skew, does it make sense to permit recalibration of
> > > the still used TSC against the marked-unstable HPET?
> > 
> > Yes, it makes sense to me. I don't know the detail of the case, if
> > the TSC frequency comes from CPUID info, a recalibration against a
> > third party HW timer like ACPI_PM should help here. 
> > 
> > A further thought is if there are really quite some case that the
> > CPUID-provided TSC frequency info is not accurate, then we may need
> > to enable the recalibration by default, and give a warning message
> > when detecting any mismatch. 
> 
> Now that you mention it, it is quite hard to choose correctly within
> the kernel.  To do it right seems to require that NTP information be
> pushed into the kernel.

Yes, we need a 'always-right' reference, but the system have to has
network access.

I know there have been many different problems related to TSC, but
the real HW/FW related problems are only about the accuracy of
TSC frequency's calibration/calculation.

Before commit b50db7095fe0 ("x86/tsc: Disable clocksource watchdog
for TSC on qualified platorms"), if the TSC freq is calculated
from CPUID or MSR, the HPET/ACPI_PM_TIMER can detect the possible
calculation problem during clocksource watchdog check. For this
case, we may need to force the recalibration by HPET/ACPI_PM_TIMER.

Thanks,
Feng

> 
> > > Second, we are very occasionally running into console messages like this:
> > > 
> > > Measured 2 cycles TSC warp between CPUs, turning off TSC clock.
> > > 
> > > This comes from check_tsc_sync_source() and indicates that one CPU's
> > > TSC read produced a later time than a later read from some other CPU.
> > > I am beginning to suspect that these can be caused by unscheduled delays
> > > in the TSC synchronization code, but figured I should ask you if you have
> > > ever seen these.  And of course, if so, what the usual causes might be.
> > 
> > I haven't seen this error myself or got similar reports. Usually it
> > should be easy to detect once happened, as falling back to HPET
> > will trigger obvious performance degradation.
> 
> And that is exactly what happened.  ;-)
> 
> > Could you give more detail about when and how it happens, and the
> > HW info like how many sockets the platform has. 
> 
> We are in early days, so I am checking for other experiences.
> 
> > CC Thomas, Waiman, as they discussed simliar case here:
> > https://lore.kernel.org/lkml/87h76ew3sb.ffs@tglx/T/#md4d0a88fb708391654e78312ffa75b481690699f
> 
> Fun!  ;-)
> 
> 							Thanx, Paul